Atmosphere breaks my ES6 package

When using my package, dalgard:viewmodel, in a project that refers to a local version of it, everything works fine, no matter whether the project uses Meteor 1.1.x or 1.2.x.

But if the package is loaded from Atmosphere, it only works with the 1.2.x version – the previous version results in dependencies not being injected.

This is the error in the console:

Uncaught ReferenceError: ReactiveVar is not defined . . . . . . . viewmodel.es6.js.js?..

What I notice here is that the file name now ends with .es6.js.js. Could it be that Atmosphere somehow compiles the bundle wrong?

Here’s an excerpt of my package.js file:

api.versionsFrom("METEOR@1.1.0.3");
api.use("grigio:babel@0.1.7");

api.addFiles("lib/viewmodel.es6.js", "client");

Everything worked fine until very recently. The issue that alerted me to the problem can be found here.

What gives?